In recent years, fuel cells have attracted much attention as next generation energy sources. In particular, polymer electrolyte fuel cells (PEFCs) using a proton conducting polymer membrane as an electrolyte membrane have high energy density, and are expected to be used in a wide range of applications such as home cogeneration systems, power sources for mobile devices, and power sources for automobiles. An electrolyte membrane for a PEFC is required to serve not only as an electrolyte for conducting protons between a fuel electrode and an air electrode but also as a partition for separating a fuel supplied to the fuel electrode and oxygen (air) supplied to the air electrode. If either one of these functions as an electrolyte and a partition is inadequate, the power generation efficiency of the fuel cell decreases. Therefore, there is a demand for polymer electrolyte membranes having high proton conductivity, electrochemical stability and mechanical strength, and low permeability to fuels and oxygen (air).
Currently, membranes made of fluorinated polymers typified by perfluorocarbon sulfonic acid (for example, “Nafion (registered trademark)” manufactured by DuPont) are widely used as electrolyte membranes for PEFCs. Perfluorocarbon sulfonic acid has a sulfonic acid group as a proton conductive group. Fluorinated polymer electrolyte membranes have high electrochemical stability, but they are very expensive because fluorinated polymers are not available for general use and their synthesis processes are complicated. The high cost of such electrolyte membranes is a major obstacle to the practical use of PEFCs. Direct methanol fuel cells (DMFCs) are a type of PEFCs in which a solution containing methanol is supplied to a fuel electrode, and there is an increasing interest in their potential practical applications because they are superior in terms of ease of fuel supply and portability. However, fluorinated polymer electrolyte membranes are highly permeable to methanol, which makes them difficult to use in DMFCs.
As an alternative to such fluorinated polymer electrolyte membranes, hydrocarbon polymer electrolyte membranes are being developed. A resin material for hydrocarbon polymer electrolyte membranes is less expensive than fluorinated polymer materials, so the use of this resin is expected to reduce the cost of PEFCs.
JP 2000-510511 T discloses, as a hydrocarbon polymer electrolyte membrane, a polyimide-based polymer electrolyte membrane containing a polyimide formed by polycondensation of a tetracarboxylic dianhydride, an aromatic diamine having a proton conductive group, and an aromatic diamine having no proton conductive group. JP 2000-510511 T describes that this electrolyte membrane has high mechanical and electrochemical stability and can be produced at lower cost than fluorinated polymer electrolyte membranes. However, JP 2000-510511 T does not consider the resistance to methanol crossover (i.e., methanol barrier property) of electrolyte membranes, and the methanol crossover resistance of the electrolyte membrane disclosed in this publication is not very high.
JP 2003-68326 A also discloses a similar polyimide-based polymer electrolyte membrane. In JP 2003-68326 A, an attempt is made to overcome a disadvantage of imide bonds which are readily hydrolyzed, so as to produce a polyimide-based polymer electrolyte membrane having high resistance to hydrolysis (long-term water resistance). However, the technique of JP 2003-68326 A also does not consider the resistance to methanol crossover of electrolyte membranes, and the methanol crossover resistance of the electrolyte membrane disclosed in this publication is not very high.
Examples of diamine components used to form polyimides include sulfonic acid group-containing diamines such as 9,9-bis(3,5-dimethyl-4-aminophenyl)fluorene-2,7-disulfonic acid, 9,9-bis(3-methoxy-4-aminophenyl)-fluorene-2,7-disulfonic acid, and 9,9-bis(3-fluoro-4-aminophenyl)fluorene-2,7-disulfonic acid (see Patent Literature 2). All of these sulfonic acid group-containing diamines have a molecular structure in which a substituent having an amino group is bonded to the carbon atom at the 9-position of a fluorene skeleton. Conventionally, many of these diamines have been synthesized and marketed because the carbon atom at the 9-position of the fluorene skeleton is a carbon atom of a methylene group and has higher reactivity than the other carbon atoms in this skeleton. Hereinafter, the 1 to 9-positions of the fluorene skeleton may be simply referred to as “the 1-position” to “the 9-position”, respectively, by omitting the phrase “of the fluorene skeleton”.